1. Field of the Invention
The present invention relates to dot formation control for a recording element that performs scanning on a sheet-like recording medium to form dots of different colors on a surface of the recording medium.
2. Description of the Related Art
To improve recording performances including recording speed, an inkjet type recording apparatus includes a nozzle group of a plurality of ink discharge ports (nozzles) assembled in a predetermined pattern and capable of discharging an ink droplet of a predetermined color at a predetermined density. Further, to improve image quality, an inkjet type recording apparatus may include a nozzle group capable of changing the density of ink to be discharged from respective nozzles. Furthermore, another inkjet type recording apparatus may include a nozzle group capable of changing in a stepwise fashion an amount of ink of a predetermined color discharged from respective nozzles at a predetermined density.
An error diffusion method is known as a quantization processing method for converting multi-valued input image data into a binary image (or an N-value image: N is a gradation number equal to or greater than 2 and smaller than a gradation number of the input image data) to control dots to be recorded by a recording apparatus. The error diffusion method includes diffusing a quantization error caused in a target pixel to a plurality of pixels surrounding the target pixel to realize a pseudo-gradational expression.
As a method different from the error diffusion method, a dither method is usable to convert multi-valued input image data into a binary (or N-value) image. The dither method includes quantizing a multi-valued input data by comparing the input data with a threshold matrix prepared beforehand to realize a pseudo-gradational expression. The dither method is simple in processing compared to the error diffusion method. Therefore, the dither method can realize speedy processing.
As discussed in U.S. Pat. No. 6,511,143, when an image quantized by the above-described error diffusion method or the dither method is actually formed on a recording medium, it is useful to determine an image formation order and a dot layout. The above-described conventional technique can eliminate deterioration in image quality (e.g., unevenness in density) by applying the error diffusion method in each scanning, even in a case where registration in each scanning is changed.
More specifically, a recording head performs main scanning operations two or more times in a predetermined main scanning recording area of a predetermined recording medium using different nozzle groups. In each main scanning operation, a binary (or N-value) image is formed using the error diffusion method. In a case where binary images are generated in this manner by performing the error diffusion method in each main scanning operation, dispersion of dots in the main scanning operation can be improved. Accordingly, granularity does not change even in a case where a physical registration (e.g., recording medium feeding amount, recording element position, etc.) is changed when the main scanning is performed plural times to form an image. Further, there is less correlation in dot layout between two or more scanning operations. Therefore, even when the registration is changed, variation in dot coverage rate relative to a recording medium surface can be reduced and also unevenness in density can be reduced.
However, according to the conventional technique, presence of a plurality of scanning directions (e.g., a forward path and a rearward path in a reciprocal printing) is not taken into consideration. Therefore, if a dot layout method is inappropriate, neighboring dots may be positioned very closely when the scanning operation is performed plural times in different directions (e.g., in the forward path and in the rearward path), bleeding of different color inks may occur.